1. Field of the Invention
The present invention relates generally to medical diagnosis and more particularly to the non-destructive testing and non-invasive examination of soft-tissue and body organs using ultrasonic diagnostic equipment. Specifically this invention is directed to a test object used to test the accuracy of and calibrate ultrasonic diagnostic equipment, such as pulse echo body scanners and the like, presently used by many hospitals and doctors.
2. Description of the Prior Art
Apparatus and techniques which permit the nondestructive testing and non-invasive examination of soft tissue and body organs are of particular interest to the medical community. Examples of presently available techniques include x-ray, nuclear medicine, thermography and diagnostic ultrasound. Ultrasonic diagnostic techniques are important because they offer a very high benefit to risk ratio for the patient and the ability to perform quality imaging of soft tissue organs. Thus ultrasonic diagnosis has found widespread applicability to the medical subfields of obstetrics, gynecology, cardiology, neurology, ophthalmology and urology among others. Ultrasonic diagnostic has proved of particular value as a diagnostic aid for the pregnant uterus including fetus and placenta, eye, breast, brain, lung, kidney, liver, gall bladder, bile ducts, pancreas, spleen, heart and blood vessels and soft tissues of extremities of neck including thyroid and parathyroid glands.
Ultrasonic diagnostic instruments operate on either a pulse-echo or Doppler principle. These principles are both well known. Most frequently the imaging of soft body tissue is accomplished using the pulse-echo principle. Short bursts of ultrasonic energy are transmitted into the body and the echoes are recorded. The time required for an emitted pulse to return as an echo provides an indication of the distance of a measured structure. Echoes occur at the boundaries between different tissues within the body since a fraction of the incident energy is deflected whenever the characteristic impedance of the structure under examination changes. Typically a change in the characteristic impedance occurs at such a boundary. Impedance is defined as the product of the density of the tissue multiplied by the velocity of sound. The first boundary will not typically reflect all the incident energy which may be reflected at subsequent boundaries. Thus, various boundaries at various depths can be observed.
Ultrasonic diagnostic equipment is used by a process called scanning. Scanning involves the movement of a pulsed sound beam propagated by a transducer through a plane. The transducer converts electrical signals into acoustic pulses. Through scanning a two-dimensional image of the various organs or body regions of interest are generated.
The quality of the two-dimensional image generated through the scanning process is dependent on the axial, lateral and elevational resolution of the transmitted ultrasonic beam and the absence or presence of side lobes. Resolution is also substantially dependent on the cross-section of the ultrasonic beam at various depths.
A known method of measuring the resolution of an ultrasonic beam employs measurement of the intensity of an ultrasonic beam using a hydrophone in an open tank of water at various depths in an engineering laboratory setting. This is impractical and not available in most clinical laboratories. For this reason, simple test objects have been devised in recent years to measure beam parameters.
A method and device for determining the axial and lateral resolution of the ultrasonic beam and the beam width in the direction of the scanning plan was adopted by the American Institute of Ultrasound in Medicine (AIUM) in 1974. The device involves the use of scanning targets immersed in an air-free scanning medium whose velocity of sound substantially corresponds to that of the soft body tissue being examined. Typically, the velocity of sound in human tissue is 1540 m/s. The construction and use of this device is fully described in chapters 7 and 9.2 of a Department of Health and Human Services publication, FDA 818139, in its Bureau of Radiological Health Quality Assurance Series entitled "Quality Assurance in Diagnostic Ultrasound Manual for the Clinical User" authored by Albert Goldstein, Ph.D. and published in October, 1980. This document in its entirety is incorporated herein by reference.
Trimmer (U.S. Pat. No. 4,417,582) provides a device and method for measuring lateral resolution in the scanning plane which differ from the AIUM test object in that it allows for continuous measurements of lateral resolution rather than the discrete measurements that characterize the AIUM test object. Clayman (U.S. Pat. No. 4,453,408) describes a device that produces a profile of the beam, thus providing for a continuous range of beam width measurements.
All of devices above are inadequate for determining the width of the ultrasonic beam in a direction substantially normal to the scanning plane, sometimes referred to as "out-of-plane beam width" or "elevational resolution" or "slice thickness," an important measurement when the ultrasonic diagnostic equipment being used generates a noncircular beam cross-section. Additionally these devices do not provide for a method to determine the existence, location, width or intensity of side lobes in either substantially the direction of the scanning plane or in the direction substantially normal to the scanning plane.